As its name implies, intraluminal stents are devices which are implantable within a body lumen for treating abnormal conditions within the lumen. For example, these devices have found use in maintaining the patency of collapsing and partially occluded blood vessels, particularly to prevent acute closure and restenosis after a vessel has been enlarged by a percutaneous transluminal coronary angioplasty procedure. These devices have also been used to reinforce other body lumens, such as the urinary tract, the bile tract, the intestinal tract and the like. They have found further use as fixation devices for holding intraluminal prosthetic grafts open and in place in the repair of weakened or abnormally dilated portions of a blood vessel.
Conventional stents are formed from a wire or the like which has been bent back and forth in a generally zig-zag pattern in a longitudinal direction and then wound in a circumferential direction transverse to the longitudinal direction to form one or more loops of a predetermined circumference. Typically, the stent is radially expandable from a collapsed condition in which the circumference of the stent is minimized so that the stent can be delivered intraluminally, to an expanded condition in which the circumference of the stent approaches the predetermined circumference to support and reinforce the lumen. The stent is normally held in the collapsed condition by a catheter during intraluminal delivery to the repair site. Once properly located, the stent is removed from the catheter and radially expanded until its circumference firmly contacts the interior wall of the lumen to hold the stent in this implanted location. This radial expansion of the stent may be effected by the dilation of an angioplasty balloon placed axially within the stent. Alternatively, the stent may be made from a shape memory metal, whereby the stent will automatically assume its expanded circumference as its temperature increases upon implantation at the desired location.
Regardless of the mechanism by which the stent is placed in its expanded condition, an important attribute of the stent is its ability to provide radial support. This capability is a concern not only where the stent is being used to maintain the patency of the lumen in which it is located, but also where the stent is being used in conjunction with a prosthetic graft to keep the graft open and to hold it at the location at which it was implanted. The ability of the stent to provide this radial support, particularly over long periods of time, is directly related to the hoop strength which the stent exhibits. For conventional stents having a generally zig-zag configuration, the hoop strength depends primarily upon the number of bends along the circumference of the stent, the elastic properties of the wire from which the stent is formed, and the diameter of the wire. Currently available stents generally have a sufficient hoop strength for use in small caliber vessels and the like because they require a relatively small number of bends along their circumference. However, where the stents have a larger circumference for use in larger caliber vessels, such as the aorta, they include a much larger number of bends and thus exhibit a lower hoop strength which is generally insufficient to maintain the patency of these larger lumens and to fix larger circumference grafts in place therein. A simple approach to increasing the hoop strength of these stents without changing the material from which they are formed is to form the stent from a larger diameter wire. Although this approach may produce satisfactory hoop strengths, it has the negative affect of increasing the bulk of the stent and thus contributes to delivery problems.
There therefore exists a need for a stent having sufficient hoop strength to provide long term radial support and graft fixation in large caliber arteries and other body lumens. More particularly, there exists a need for a stent which exhibits this high hoop strength with little to no increase in bulk over conventional stents and which therefore does not contribute to difficulties in intraluminal delivery.